This invention relates to process controls, the collection of process data, maintenance of a process data archive, and easy access and display of the data when and where needed.
Industrial processes often use automatic controls to reduce costs, improve quality, and increase productivity. When process difficulties are encountered or improvements are needed it is necessary to obtain reliable data on previous process history for analysis or diagnosis. This can be difficult to do since the industrial environment is often hostile and damaging to data collection equipment. Customarily persons wanting process data attach data logging equipment to the process. Sometimes the automatic controllers make provision for external data logging attachments. Data loggers record data at some fixed periodic rate. The user must choose an appropriate sample rate to collect the desired data. If he wants to know about some action which occurs for a few seconds, he might sample at a once per second rate. Many of the data points will be uninteresting since they occur between the desired process events under study. Most of the recorded data may simply report that nothing has changed. The available memory can be filled quickly with uninteresting data. Sometimes a specific predetermined event such as machine startup is used to start or stop data collection in an attempt to collect only relevant data. It is difficult to predetermine which events will always cause the important data to be collected. The process failures of interest are by their nature typically random and unpredictable. A further difficulty with data loggers is that they often have to be taken somewhere away from the process to have their contents read and analyzed. This procedure prevents the data from being immediately available for diagnostic procedures at the process under control.
Another disadvantage of conventional data logging is that the user must anticipate the need for the process data and have the data logger working before the critical data occurs. Often it is unknown when a process will have a problem and data is not collected at the critical time. A further disadvantage with conventional data logging is that manual handling can result in data loss or corruption.
Another approach to the process data collection problem is to control the process with a full general purpose computer with extensive data storage and data backup capability. The hostile factory environment makes it difficult and expensive to isolate and protect the expensive computer components if it is conveniently located near the process being controlled. If the computer is located some safe distance from the process then the needed data will also be some distance away.
It is the object of the present invention to simply and economically add selective process data logging to process controls and provide a way to instantly replay desired process data on the at the process site. Since the data collection is incorporated within the controller it is possible to configure it to collect data selectively and record only when there has been some change in the process. Only significant process events are recorded and memory space is not wasted recording uninteresting repetitive data. This economic use of storage can permit several months of data to be stored within the controller in a relatively modest size memory. Another advantage is that the process event storage is always connected and active when the controller is on. This guarantees that if some unexpected process event occurs, it will be recorded. A very troublesome unexpected process event is the intermittent failure which occurs, ruins product, and then disappears spontaneously. Without a continuous record of process changes it can be extremely difficult to locate and fix the source of such failures. Further, since the data storage is within the controller and not external to it; tamper resistance and security are improved compared to a conventional external data logger. Further, since only a modest addition to the usual rugged process controller is needed, the expense and complexity associated with a full general purpose computer is avoided.